Conventionally, three basic types of deep foundations have been used in the construction industry. The first such type is the driven pile, which is typically manufactured off-site and transported to the construction site, where it is then driven into the ground. Driven piles can be made from a variety of different materials and in a variety of different shapes. These include the pre-cast concrete square pile, wood pile, steel "H" pile, steel pipe pile, and mandrel-driven step tapered piles. A conventional mandrel-driven tapered pile displaces ground below it as it is driven into the ground, and is then filled with ready-mix concrete. A common size for pre-cast piles is 16 inches square in cross-section, and often multiple such driven piles are grouped together and topped by a cap for supporting the load presented by the remainder of the foundation and the overlying structure.
A second type of conventional pile is made from drilled shafts. Drilled shafts are drilled excavations which are filled with reinforcing steel cages and concrete. Drilled shaft diameters are typically large (e.g., 18 inches to 72 inches or more), and they are usually poured to the surface of the existing grade, since no cap is required. Drilled shafts are also used to form barriers when installed in the form of secant wall piles, wherein adajcent drilled shafts are positioned so that they intersect along one side of their outer diameters. Such barriers may be used to prevent the migration of soil contaminants or moisture past a boundary defined by the secant wall piles.
A third type of conventional pile is auger cast piling, which has characteristics of both drilled shafts and driven piling. Auger cast piles are continuous auger flight excavated piling. As the continuous flight auger is retracted, a cement grout is added through the auger to fill the excavation. Steel reinforcing, typically in the form of a steel cage or a single steel reinforcing bar, is then added. Auger cast is usually used in soft ground conditions.
The selection of the type of deep foundation to be used is typically based on numerous factors. Chief among these factors are the geologic characteristics of the ground in which the foundation is to be placed. The hardness of the ground, the moisture content, and the presence of rocks are all characteristics which are often taken into consideration. For instance, in harder ground, usually drilled shafts are used. In softer grounds, usually the driven pilings are used.
Each of these conventional piles has certain disadvantages. Driving piling, for example, causes vibration during installation. This vibration may cause damage to nearby structures. Furthermore, the noise attendant with driving piling often makes it an unacceptable foundation system for constructions near populated urban areas. A further disadvantage of driven piling is that most such piles are fabricated offsite, necessitating their transportation to the job site. Such transportation can be expensive, especially when the job site is in a remote area.
Large-diameter drilled shafts also have numerous disadvantages. A principal disadvantage is the low ratio of surface area to volume of material. Deep foundations are typically designed to maximize skin friction (which is proportional to the external surface area of the pile or group of piles) relative to the volume of material required to construct the piles. Piling elements of relatively smaller cross-section, such as most driven piling and auger cast piling, have more skin friction per unit volume of material (concrete and reinforcing steel) than a drilled shaft. For example, four 18-inch diameter piles have the same skin friction value as one 72-inch diameter drilled shaft, yet use only 25% of the volume of concrete and reinforcing steel required for the larger-diameter drilled shaft.
Drilled shafts have the further disadvantage that, in engineering assessments, they are often assigned no end bearing capabilities. The bottoms of drilled shafts are often difficult to inspect for cleanliness, soil characteristics, and other indicia of end bearing capabilities. Consequently, drilled shafts are typically assigned little, if any, end bearing capabilities.
Drilled shafts and auger cast also share the disadvantage that they are time dependent on the timely delivery of the cementitious material which will be placed to form the pile. Waiting for delivery of the material can result in costly and inconvenient schedule disruptions and delays.
Drilled shaft and auger cast share the further disadvantage that during installation large volumes of spoil dirt are brought to the surface. Because these piles require excavation, large volumes of dirt, rocks and other earthen material are displaced and must be removed from the construction site. Often, this earthen material is contaminated with hazardous chemicals and the like, and disposal of the contaminated refuse may be difficult or impossible. Where sub-surface contamination is known to exist, the use of drilled shafts and auger cast may often be avoided so as not to make the problem worse by creating a surface contamination. Even clean spoil dirt removed form the excavation and brought to the surface has to be disposed of, and such disposal is costly even if no contaminants are present.